Linking Error - Use nm to fix

Question

I have a linking error which I'm supposed to fix using the nm command on Unix.

If I have the following linking error: undefined reference to 'program_name', and when running nm on that object file, program_name symbol is shown as follows in the symbol table: U program_name.

I know program_name is undefined, and is defined in another object file which needs to be included with the original object file. My question is: how can I find which object file it is? Is that possible? I have a bunch of object files in a directory and it would be one of them. There's way too many to try all of them.

Answer 1

You can do this:

$ nm *.o 

Look for program_name in the output (assuming you have a .o extension on your object files).

Answer 2

Yes. Instead of a U in the output, you'll find a T (for "text segment") in the object file that has the symbol defined. Something like this should work (in bash):

for f in *.o; do if (nm "$f" | grep 'T program_name'); then echo "$f" matches; fi; done

Here, I'm looping over all *.o files, and whichever ones have the symbol defined get their filenames printed out.

Answer 3

Try nm -o *.o |grep program_name and look for the .o file where the symbol has a type of "T" (assuming it's a function name). the -o option to nm prefixes each line with the filename. Depending on your os, nm may or may not have this option (it might also be -A) so check your manpages. If it's not there at all, then you'll need to write a shell for loop, something like

for obj in *.obj; do 
  echo $obj:
  nm $obj | grep program_name
done`